The production of carboxylic acids by the carbonylation of alcohols with carbon monoxide is well known. One example, the carbonylation of methanol, is the most important reaction for the commercial production of acetic acid. Acetic acid, which is a principal ingredient in vinegar, has hundreds of uses in addition to giving flavor to cooking and salads. Paints and adhesives contain latex emulsion resins polymerized from vinyl acetate monomer (VAM) made with acetic acid, and some cellulosic fibers and plastics are manufactured from acetic anhydride derived from acetic acid.
Both homogeneous and heterogeneous catalysts for the carbonylation of methanol to acetic acid are known. Although homogeneous catalysts are well known, they cause numerous problems. U.S. Pat. No. 5,334,755 (Yoneda et al.) lists the problems associated with a homogeneous rhodium compound-methyl iodide catalyst dissolved in acetic acid, including primarily that overly large reactors and reactor-related equipment are needed due to the low solubility of the rhodium compound in acetic acid. Other problems with homogeneous catalysts include that operating costs are relatively high when low water levels are employed and excessive hydrogen iodide corrosion occurs when high water levels are used. Heterogeneous catalysts, on the other hand, have as a primary advantage over homogeneous catalysts the fact that smaller reactors can be used. Heterogeneous catalysts are disclosed in U.S. Pat. No. 4,328,125 (Drago et al.); U.S. Pat. No. 5,155,261 (Marston et al.); U.S. Pat. No. 5,281,359 (Scates et al.); U.S. Pat. No. 5,334,755 (Yoneda et al.); U.S. Pat. No. 5,364,963 (Minami et al.); and U.S. Pat. No. 5,466,874 (Scates et al.).
One of the problems with heterogeneous catalysts is that a heterogeneous catalyst can exit the reaction zone with the reaction effluent and settle out or otherwise accumulate in downstream equipment. Another problem with heterogeneous catalysts is that a heterogeneous catalyst can undergo some attrition during its useful life in catalyzing carbonylation reactions. To clarify, it is helpful to define the term "attrited solid catalyst particles" as used herein. "Attrited solid catalyst particles" means solid catalyst particles that have undergone mechanical attrition and/or thermal attrition. Mechanical attrition means the transformation of larger catalyst particles into smaller catalyst particles in the absence of a change in the chemical composition of the catalyst particles. Examples of mechanical attrition include breakage and fracturing, which can yield chips, pieces, fines, or other fragments of the solid catalyst particles. Most commonly, mechanical attrition of catalyst particles occurs during transport or other movement of the catalyst particles. Transport of the catalyst particles subjects the catalyst particles to erosion and abrasion as a result of contact with other catalyst particles as well as with the inner surfaces and edges of vessels and conduits.
Thermal attrition means the transformation of catalyst particles that occurs as a result of exposure to elevated temperature and that does result in a change in the chemical composition of the catalyst particles. Polyvinylpyridine resin, which is a common carbonylation catalyst, serves to illustrate two examples of thermal attrition, namely first the breakdown of the cross-linkages within the polyvinylpyridine resin, and second the breakdown of the linkages comprising the backbone of the polyvinylpyridine resin. It should be pointed out, however, that the term "thermal attrition," in the case of resins, is meant to exclude the plasticization or melting of the resin, the decomposition of the resin into a gum, the decomposition of the resin into a soft coke, and the complete depolymerization of the resin. Although polyvinylpyridine resins at certain conditions can melt and can undergo such decomposition and complete depolymerization reactions, these reactions are deleterious to the select properties of the polyvinylpyridine resins to catalyze carbonylation, and therefore carbonylation processes are generally operated outside the range of conditions at which such decomposition and depolymerization reactions occur.
The problem with attrited solid catalyst particles is that attrited solid catalyst particles tend to severely foul equipment in which the phase of the bulk fluid that carries the attrited solid catalyst particles changes from a liquid to vapor. When a liquid-solid reaction effluent that contains attrited solid catalyst particles flows through equipment such as flash drums and heat exchangers wherein the liquid phase is at least partially vaporized, the attrited solid catalyst particles can cluster or settle and compress into deposits which are difficult to remove or which with heat input can form a tar-like deposit. Such deposits can interfere with the performance of this equipment. Solid catalyst particles, in turn, can stick or adhere to the tar-like deposit, thereby adding to the quantity of the deposits.
Tar-like deposits can become even more problematic when the attrited solid catalyst particles combine with heavy by-products of side reactions that occur during carbonylation. As used herein, the term "heavy by-product" means a product of the reaction zone that has one more carbon atom than that of the desired carboxylic acid product. In the production of acetic acid, for example, propionic acid and other compounds having three or more carbon atoms are considered to be heavy by-products. Generally, at separation conditions, a heavy by-product has a boiling point that is greater than that of the desired carboxylic acid product. Thus, at normal conditions, propionic acid boils at about 141.degree. C. while acetic acid boils at 118.degree. C. Of course, even in the absence of attrited solid catalyst particles, heavy by-products alone can foul or otherwise interfere with the operation of downstream equipment.
Accordingly, methods are sought that prevent or at least alleviate the fouling problems that can arise as a result of the use of heterogeneous catalysts in carbonylation processes, in order that the benefits associated with the use of heterogeneous catalysts can be more fully realized.